The methylation of inorganic arsenic to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) have been generally considered to be the major pathway for inorganic arsenic biotransformation and detoxification. Yet, when arsenate/arsenite is injected into the Callithrix jacchus (marmoset) monkey or chimpanzee, monomethylarsonic acid and dimethylarsinic acid are not found in the urine. With the development of a rapid assay for the methyltransferases of arsenic metabolism, we have investigated the methyltransferases of the marmoset monkey liver. We have found that the marmoset, a New World animal, is deficient in liver arsenite and monomethylarsonic acid methyltransferase activities. However, the rhesus monkey, an Old World animal, has ample amounts of such methyltransferase activities. The tamarin, another New World species, is also deficient in these methyltransferases. Polymorphism and deficiency of these methyltransferases may have allowed high levels of arsenite to be maintained in the blood and liver of the marmoset and tamarin. Such high levels of arsenite may have been selective for survival of the species. The rhesus liver methyltransferases for arsenite and MMA have been purified and found to have some properties different from those of the previously reported purified rabbit liver activities. The rhesus and rabbit liver arsenite and MMA methyltransferases are devoid of catechol O-methyltransferase activity.